1. Field of the Invention
The present invention relates to aqueous hair and skin cleaning compositions like shampoos, shower gels, bath gels or generally aqueous cosmetic cleaning liquids having high foaming ability, for which an increased viscosity is necessary or desired for application reasons.
2. Discussion of the Background
Aqueous hair and skin cleaning compositions have in the past been predominantly based on anionic surfactants, such as, in particular, fatty alcohol sulfates or fatty alcohol ether sulfates, if desired, in combination with fatty acid amides, i.e. surfactant combinations, which can be readily thickened by means of electrolytes. As alternatives, carbobetaines and sulfosuccinates have recently been included for reasons of higher skin compatibility. Suitable thickeners for these systems are likewise fatty alcohol ethoxylates having a relatively low degree of ethoxylation (see: A. Behler et al., SOFW, vol. 116, p. 60 (1990)) or alkyl polyglycosides (see: EP 0 070 074 and EP 0 384 983). Furthermore, in some cases water-soluble polymers are added as thickeners to the predominantly anionic surfactants.
In all of these formulations, only data on viscosity have entered into the discussion of the rheology in the literature; values and limitations for any elasticity present have, in contrast, not been mentioned.
Viscoelastic surfactant solutions (see: H. Hoffmann and H. Rehage, in Surfactant Solutions, Surfactant Science Series, vol. 22, R. Zana, Ed., New York, 1987, p. 209 ff.), i.e., systems which have both prominent viscous and elastic properties, which when supplied with mechanical energy therefore respond both with conversion into heat and storage of mechanical energy, have been studied primarily in the case of quaternary ammonium compounds in the presence of strongly binding counterions. The example of cetylpyridinium salicylate is typical. However, other cationic surfactants too, such as hexadecyltrimethylammonium bromide in the presence of extremely high salt concentrations, show viscoelastic behavior (see: A. Khatory et al, Langmuir, vol. 9, p. 1456 (1993)). Such properties of surfactants are used in special cleaners which are to be transported as undiluted by static water as possible to their site of action, such as, for example, low-surfactant, high-alkali and high-electrolyte pipe cleaners (see: EP 0 317 066). In other cases, the viscoelastic behaviour of cleaning solutions is enhanced by adding strongly crosslinked polymers which themselves are significantly elastic (see: EP 0 398 021 and EP 0 584 877). In the case of simple surfactant solutions, such as, in particular, the anionic surfactants which are so important in applications, viscoelastic behaviour has not hitherto been described. For the purposes of the present invention, surfactant solutions are liquids which can also contain dispersed materials.
The viscoelastic effects in the flow behaviour of aqueous personal care solutions have considerable importance for their use. Two processes in particular, namely taking the product from the container and its distribution on the skin or the hair, are relevant here. While the first process usually occurs at relatively low shear rates of up to about 10 s.sup.-1, the distribution on the skin or the hair occurs under the action of substantially higher forces, usually at shear rates above 100 s.sup.-1. The discussion which hitherto rested exclusively on viscosity data therefore promoted shear-thinning flow behaviour, by means of which slow flow from the container, simple metering combined with the effect of high active ingredient concentration, and also simple distribution on the skin are ensured. However, since shear-thinning surfactant systems frequently show viscoelastic effects and the elastic contributions usually increase strongly with rising forces, the discussion hitherto is insufficient and has to be corrected to take viscoelasticity into account. Accordingly, the elastic contributions must not become too high, since otherwise satisfactory distribution on the skin or the hair is made more difficult. In addition, high elasticity leads, during outflow of the liquid, to a relatively unappealing oscillation of the shapelessly thick liquid thread in the neck of the container; the liquid flows back if the inclination of the container is reduced only slightly. On the other hand, systems which are exclusively viscous tend to form very long rejuvenating threads, which in turn could form cobwebs, a behavior which is likewise very unsatisfactory aesthetically. Optimum flow behaviour of the liquid can therefore only be expected when viscosity and elasticity of the liquid are matched to one another.
Thus, there remains a need for aqueous viscoelastic surfactant solutions for hair and skin cleaning which exhibit good flow behavior and good distribution on the skin and hair.